This application is a 371 of PCT/JP99/03128, filed on Jun. 10, 1999.
The present invention relates to a method for curing ultraviolet-curing resin coated on a streaked substance and an ultraviolet curing apparatus used in the method.
Methods in which an ultraviolet-curing resin is coated on a linear body such as an optical fiber, a copper wire, a fiber thread, or on a linear body in which a plurality of linear bodies are bundled together or arranged parallel to each other, and is then cured by irradiation of ultraviolet rays, are widely used as methods for coating linear body.
The method disclosed in Japanese Patent Publication No. 6-84015 can be given as an example. To explain this method with reference to FIG. 9, a quartz tube 38 is placed inside a curing apparatus 21 as a tubular member for transmitting ultraviolet rays (hereinafter called xe2x80x9cultraviolet-transmitting tubular memberxe2x80x9d), and a linear body 41 coated with ultraviolet-curing resin is passed through the quartz tube 38, and the ultraviolet-curing resin coated on the linear body is cured by irradiation of ultraviolet rays from an ultraviolet lamp 24 outside the quartz tube. At that time, the gas inside the quartz tube 38 is purged by the flow of inert gas the flow rate of which is controlled by a flow-rate-regulating valve 40 and which is simultaneously exhausted therefrom with a suction pump 29. Despite this, some of the ultraviolet-curing resin vaporizes and adheres to the inner surface of the quartz tube. Therefore, dimming of the quartz tube 38 increases as the operation proceeds and the ultraviolet transmittance of the quartz tube declines. As a result, the curing degree of the ultraviolet-curing resin declines because the ultraviolet rays reaching the linear body decreases until eventually quality problems arise.
In order to avoid this problem, the ultraviolet dosage transmitted through the quartz tube is measured and the quartz tube is exchanged for a quartz tube with no dimming when the ultraviolet dosage decreases to the lower limit at which the ultraviolet-curing resin can be sufficiently cured. Actually, the quartz tube is exchanged at the point when curing of the ultraviolet-curing resin of a linear body is completed, if it is foreseeable, by measuring the ultraviolet rays transmitted through the wall of the quartz tube, that the transmission of ultraviolet rays through the wall of the quartz tube will become insufficient for the curing operation of the ultraviolet-curing resin of the next linear body.
Further, in Japanese Patent Application Laid-Open No. 10-59749 a method is disclosed in which the inside surface of a quartz tube is coated with titanium dioxide such that it is difficult for the ultraviolet-curing resin to adhere to the quartz tube.
The disclosure of the published specification, which uses the terms slightly different from those used in the present application, is explained in the terms used herein.
FIG. 8 shows a vertical cross-section of the apparatus disclosed in Japanese Patent Application Laid-Open No. 10-59749. Numeral 11 indicates a coated linear body comprising an optical fiber coated with ultraviolet-curing resin, 12 a quartz tubular member allowing the transmission of ultraviolet rays, 13 an ultraviolet lamp, 14 an apparatus containing box, 15 a gas supply tube, and 16 a gas exhaust tube, respectively.
In this ultraviolet irradiating apparatus, the ultraviolet rays are irradiated to the coated linear body via the tubular member 12 from the ultraviolet lamp 13 placed on the outer side of the tubular member 12 while the coated linear body 11 is passed through the tubular member 12 and the ultraviolet-curing resin coated on the linear body is cured. During that time, nitrogen gas is supplied from the gas supply tube 15 into the tubular member 12 and exhausted therefrom through the gas exhaust tube 16.
The tubular member 12 disclosed in Japanese Patent Application Laid-Open No. 10-59749 is that one having a titanium dioxide layer of 0.5-100 xcexcm of thickness on the inner surface of a quartz tube. This titanium dioxide layer promotes the decomposition of tarry organic substances generated during the curing reaction and is intended to reduce the decline in ultraviolet transmission due to the adherence of tarry organic substances.
However, the inventors of the present invention found that despite coating the inner surface of the quartz tube with titanium dioxide as disclosed in Japanese Patent Application No. 10-59749, dimming components originating in the ultraviolet-curing resin adhere to the quartz tube and the frequency of exchanging the quartz tubes is not decreased.
Further, it also became clear that although dimming components did not adhere quite so much, a sufficient irradiation dosage could not be obtained from an ultraviolet irradiating lamp of normal usage and occasionally the ultraviolet-curing resin was undercured.
It was found that, when a quartz tube having the inner surface coated with titanium dioxide was used as disclosed in Japanese Patent Application Laid-Open No. 10-59749, the transmission coefficient of the ultraviolet rays in a wavelength region of 360 nm or shorter, which is necessary for curing a resin, fell to less than 0.5, which is comparatively less than in the case of a quartz tube without a titanium dioxide layer.
In order to obtain an ultraviolet rays dose sufficient to perform curing of the ultraviolet-curing resin, there is also a method using a high-output ultraviolet irradiating lamp, but it is not economical.
In other words, the present invention provides an ultraviolet irradiation method and an ultraviolet irradiating apparatus that solves the aforementioned problems of the prior art.
The present invention is characterized in that, in a method for applying a ultraviolet-curing resin on a linear body and curing it by irradiating ultraviolet rays from outside the ultraviolet-transmitting tubular member while passing it through an ultraviolet-transmitting tubular member placed inside a curing apparatus, the inner surface of the ultraviolet-transmitting tubular member is coated with titanium dioxide and the oxygen concentration is controlled to 0.1% or more in an atmosphere inside the ultraviolet-transmitting tubular member.
In the above method, the oxygen concentration inside the ultraviolet-transmitting tubular member can be raised to 0.1% or more by purging with a mixture of an inert gas and air or oxygen inside the ultraviolet-transmitting tubular member.
It is even better if the oxygen concentration inside the ultraviolet-transmitting tubular member is 0.5% or more.
In the above method it is even better if the thickness of coating layer including titanium dioxide is 0.03 xcexcm or more but less than 0.5 xcexcm.
The frequency of need to exchange ultraviolet-transmitting tubular members of the curing apparatus can be decreased because dimming of the ultraviolet-transmitting tubular member can be prevented by the above-described methods of the present invention, that is, by coating the inner surface of the ultraviolet-transmitting tubular member with titanium dioxide and controlling the inner atmosphere of the ultraviolet-transmitting tubular member to a designed oxygen concentration. Further, because a single operation of coating an ultraviolet-curing resin can be continued for a longer period of time, a longer linear body can be obtained, which could not have been obtained conventionally.